Mavericks: How to ADD Input Source via plists/defaults?
Turns out that the format I used for the AppleEnabledInputSources entry was bad because it considers everything as type string, but KeyboardLayout ID must be of type integer!
So the correct way of doing this would be via XML:
defaults write com.apple.HIToolbox AppleEnabledInputSources -array-add '<dict><key>InputSourceKind</key><string>Keyboard Layout</string><key>KeyboardLayout ID</key><integer>15000</integer><key>KeyboardLayout Name</key><string>USInternational-PC</string></dict>'
After running that command and doing a logout/login, looks like things work fine.